Method in carrying out treatment of melted pig iron or other alloyed iron



May 27, 1952 M. S. KALLING ET AL METHOD IN CARRY 2,598,393 ING OUT TREATMENT OF NELTEO PIG IRON OR OTHER ALLOYED IRON Filed Oct. 24, 1949 Tapia/'a7 Hole (Ittornegs.

Patented May 27, 1952 METHOD IN CARRYING OUT TREATMENT OF MELTED PIG IRON OR OTHER AL- LOYED IRON Bo Michael Sture Kalling, Domnarvet, and Oskar Lennart Lindskog, Borlange, Sweden Application October 24, 1949, Serial No. 123,304 In Sweden October 25, 1948 l V The present invention relates to the type of oxidizing treatment of pig iron or other iron alloy in molten condition, in .which the heat necessary for carrying out the oxidation and for maintainingJr the molten bath at a suitable temperature is generated by reaction between free 'oxygen and the oxidizable constituents of the 'tion of pure oxygen gas or. oxygenated air for the oxidation due to which a considerable quantity of cold scrap iron may be added to the molten bath. The heat economy is further improved by the possibility in the new process, withoutv disadvantages, completely to burn within @the furnace chamber all the carbon monoxide formed by oxidation of carbon inthe bath. 1

Another such object relates to diminishing iron losses as iron oxides caused by oxidizing iron with the oxygen added to the bath which iron oxides enter into the slag.

A further object is to reduce the evaporation of iron with the formation of fumes which can not be avoided by using gases high in oxygen in the hitherto known processes of this type and which Cause 1o claims. (o1. vs -s0) Other objects of the invention andadvantages to be derived from its use in practice will become vapparent from the following description.

In carrying out oxidizing treatments of the type under vconsideration the Bessemer process and some modifications -ofthe same have been used for a long time. r'Ihe Bessemer process comprises substantially the steps of introducing moltn'pig iron into a converter and supplying purpose besides bringing about the oxidation to create such a rapid movement in the bath that the reactions will rapidly approach equilibrium whereby the iron preliminarily oxidized by means of the oxygen will be reduced again by thegmore easily oxidized alloying constituents. The heat required to keep the iron in molten condition and in a necessary degree to increase the temperature during the treatment is in this case liberated by the reaction between supplied oxygenand the alloying constituents. l

In the Bessemer process the oxygen supply, the stirring of the bath and the heat development are entirely dependent of each other. Further as the supply of oxygen for technical reasons` has certain maximum and minimum limits the primary disadvantage of this vprocess is that the possibility of variationsof the'sarne are restricted. Thus, itis impossible to reduce the supply of oxygen below a certain limitbecause the pressure of the gas supplied must always be at least sufficiently high that the iron-is prevented from leaking through the twyers.

For improving the heat economy of the-Bessemer process it is further desirable to be able to carry out the oxidation 'by means of pure oxygen gas or a highly oxygenated air. The oxygen content of the gas, however, is limited by the ability of the furnace bottom around the twyers to withstand high temperature. VAt an oxygen content of about 40% in the supplied gas the strength of the furnace bottom begins to decrease severely due to the local high temperature occurring when the oxygen contacts the iron bath and the utilization of pure oxygen is made entirely impossible for this reason. This is a great drawback as particularly in the case of bottom blowing it is an important desideratum to keep the nitrogen content of the gas as low as possible for enabling the production of a steel lowl in nitrogen. Further when using the bottom blowing process the combusidon heat of the carbon is unsatisfactorily used because the carbon monoxide formed will be burned to carbon dioxide only after leaving the converter so vthat the heat generated can not be utilized for the process.

In the other process of this type, viz.A the vsocalled side blowing process, the oxidizing gases are not introduced through openings in the bottom of the converter but through twyers in the side Walls of the converter near the surface of the iron bath. Also in this case the gas serves to bring about the stirring necessary in the Abath and to oxidize alloying constituents; The stirring action in this case ofcourse is less because of which the reaction will not be as vcomplete as in the bottom blowing rprocess resulting in considerably increased iron losses :in the slag. The heat economy, however,isbetter than in the bottom blowing process as the gasses formed by the oxidation are completely burned when leaving the converter. Because of the less stirring action the bath is more extensively superheated locally in places where the oxygen enters into contact with the iron bath especially when using oxygenated air or pure oxygen causing the disadvantage that a considerable part of the iron and possibly also manganese is evaporated involving a substantial decrease in the iron yield and also the formation of great quantities iron fumes, which may be very annoying from a sanitary point of view. The extremely line particles of iron are very difficult to separate from the gases according to known methods for the purification of gases. In the side blowing method there is also when using oxygenated air the vsaine disadvantage as in the bottom blowing method that the lining of the twyer openings is severely attacked. Compared with the bottom blowing f methods the side blowing methods have the'advantage that the nitrogen content of the steel is less `because of the nitrogen of the gas supplied not coming into such an intimate contact with the iron.

It has been proposed to carry out oxidizing treatments of the type under consideration by 'supplying high pressure oxygen gas through twyers and the like 'directed against or introduced through the surface of the bath. By proceed- .l

ing in'this way it is possible to prevent local attacks upon the furnace lining but as this method has been carried into practice it has been impossible to obviate considerable iron losses through oxidation as well as formation of iron fuifnes.

According to the present invention the advantages mentioned above are obtained while obviating the disadvantages of the known processes by avoiding stirring action by the supply of the gas used for therening process. In the following the invention will be fully described in connection with the use of a rotating furnace for bringing about the stirring action. For bringing the invention into practice it is obvious, however, that other means may be used for stirring the bath, as e. g. ofthe mechanical or electrodynami- `cal kind.

YThe'idea of using rotating furnaces in the production of steel is not new per se as it has been proposed to use rotating furnaces when melting and treating steel. For Vthis purpose slowly rotating furnaces have been used which essentially are operated according to the open hearth process (Martin process) and in which l.

the main object of the rotation vis to facilitate the `heat transmission tothe metal which .is melted and superheatecl by blowingv high temperature combustion gases vthrough the furnace. .In the process according to the invention the main vobject is to obtain a sufficiently intense movement in the iron bath by rotating the furnace for obtaining an intense contact between the bath and the oxygen containing gas supplied for oxidizing 'the components to be oxidized 'at such va high reaction velocity that external supply. of heat is superfluous and for obtaining such 'a reaction between the bath and the slag that the content of iron oxides in the slag maybe vtre-reduced. summarizing it may be said that in the known processes the rotating furnace substantially is used for improving the heat transmission in a method, which otherwise corresponds to the open hearth processes while the invention relates to a rapid refining process substantially 'coinciding with the Bessemer process. In the first mentioned process the result aimed at is obtained through a relatively slow stirring while according to the invention a rapid stirring action is necessary as a stirring of the type according to known processes has proved to be insufficient for giving satisfactory results.

A furnace of the type referred to above is illustrated in the accompanying drawing which is an axial section of the furnace. In the drawing, l is the furnace body which is mounted for rotation on rollers 2. 3 is a tapping hole. Oxygen is supplied through pipe 4 which extends through Vtheaxial opening or tuyre 5 in one end of the furnace and is provided with the water cooling jacket 6. 1 is an outlet opening for waste gas and 8 the iiue therefor.

Although air and other gas mixtures relatively high in oxygen may be used in the process according to the invention the advantages of the process become more apparent when using gas vmixtures rich in oxygen, e. g. with oxygen contents above 50%, or pure oxygen. If the process is carried out in a stationary furnace and a gas rich in oxygen is supplied to the bath from above it is very diicult to prevent a considerable part of the iron from being oxidized before the process has gone to an end. By rotating the furnace rapidly it is, however, possible also when using pure oxygen to reduce the iron content in the slag to an acceptable amount.

The disadvantages mentioned above due to the formation of iron fumes are decreased or completely obviated when using a rotating furnace as the strong stirring action will equalize the oxygen supply to the Vbath and prevent local superheating.`

When the process is carried out in a rotating furnace it is not .necessary to supplythe oxygen with such a high pressure as in the common Bessemer process. .Also when a considerable quantity of slag is present in the furnace it is possible to obtain a rapid reaction only by rotating the furnace with a suiiicient speed of rotation. Said speed ought to be so high that through the resulting stirring action theY iron in the bath will pass through the slag so that a direct contact between the iron and the gas is obtained over a certain part of 'the surface. However, it has been found that a rapid refining action may be obtained also without such a direct contact if only the slag is sufficiently free-flowing and the slag layer is not too thick. In processes in which larger slag quantities are formed it may be desirable to pour off the slag continuously or intermittently for facilitating the reaction.

In such case the oxygen must not necessarily be supplied through a twyer directed against the bath but may preferably be supplied -axially to the furnace through a wide nozzle in a central opening in one of the end walls of the furnace. When doing so the advantage is obtained that the oxygen before it reaches the bath is mixed with the furnace gas thus avoiding local superheating and also the formation of fumes.

If when using a gas rich in oxygen it is desirable to increase the stirring action by blowing the gas under pressure against or into the bath it may be suitable to mix the gas with another gas before it contacts'the iron for reducing the formation of iron fumes. This may be done by mixing the oxygen containing gas before introducing the same into the furnace with another gas containing carbon dioxide and/or water vapor or by mixing it with reaction gases from within the ascsgaes furnace. This latter mixing may be carried out outside the furnace but it is preferably carried out within the furnace by constructing the oxygen twyer as an ejector so that the furnace gas is sucked up with the oxygen and mixed with the same before the gas jet'reaches the bath. In this case it is of course important that the circulating furnace gas does not contain combustible gases but that the combustion within the furnace is complete.

In the Ibasic Bessemer process (Thomas process) in which great quantities of burnt lime 12- 15%) are to be supplied to the bath at the beginning of the treatment it is very difficult to carry out the treatment in a stationary furnace if the oxygen is supplied from above through twyers directed toward the bath even if a high gas pressureis used. The great quantity of lime in this case prevents the gas from coming into eiective contact with the iron bath. In the rotating furnace these diiiiculties are obviated and it is possible also When using very large quantities of lime to carry out the process Without using high pressure gas.

This oxidation process with uncombined oxygen in a rotating furnace may with advantage be used both for the entire process when transforming pig iron or other alloyed iron into steel and also for certain parts of this process. The process may be carried out in acid as well as basic lined furnaces and in the later case pig iron rich in phosphorous may with advantage be used as a starting material. When using highly concentrated oxygen the heat economy of the process will be very good and the temperature conditions may be controlled as desired independently of the chemical composition of the pig' iron. A great excess of heat is always produced and may be used for simultaneously melting cold material, e. g. scrap iron or iron ore for partial refining with iron ore. Due to the rotation of the furnace during the process the combustion of carbon monoxide into carbon dioxide does not involve any risk of the temperature in the furnace lining rising to dangerous level above the bath surface.

In a rotating furnace it is further possible to carry out reactions which have been completely impossible to carry out in furnaces of other types. In carrying out the oxidizing treatment under consideration according to the known processes the reaction stops when the supply of air or oxygen is cut off. The process as a whole must be .carried out under strongly oxiding conditions. In certain cases this is a very serious drawback, e. g. when it is desirable substantially to remove a certain element without simultaneously incurring excessive iron losses. When using a rotating furnace it is possible to carry out the last stage of the process in a neutral atmosphere by disconnecting the oxygen supply at a suitable moment and continuing the rotation until the reaction between the iron bath and the slag has reached the desired extent. l

This procedure may be carried out several times during the treatment for transforming pig iron into steel. If it is considered desirable to draw off the slag from the furnace at one orgseveral times it is preferable to carry through such a rotation in neutral atmosphere before each drawing off of the slag takes place and this mainly for decreasing the iron content of the slag and the coincidental iron losses. Especially when ending the process in connection with finishing the steel such a treatment may be desirable also for decreasing the carbon content and under certain circumstances also the oxygen content of the steel. This stirring in neutral atmosphere is also very important for effectively removing phosphorous in a furnace with basic lining, as it will be possible to decrease the phosphorus content to harmless amounts without any appreciable increase in the iron losses even when using a starting materialrich in phosphorous. If vthe slag is drawn 01T when the main part of the phosphorous is chemically bound in the slag and by adding lime and possibly also some iron ore and continuing the process without supplying oxygen at least during the end period it is possible to obtain a high degree of removal of phosphorous. It is preferable to keep the bath at such elevated temperature that it is possible to obviate supply of oxygen after the slag is drawn off. In the process according to the invention it is also pos-v sible to obtain a slag rich in phosphorous even when the pig iron is low in phosphorous which is diincult to achieve when using a basic Bessemer process or a refining process using ore.

According to the invention it is also possible to cbtain effective removal of sulphur in the Besse# merv treatment especially when the process is carried out in a furnace with basic lining. If lime is added at the beginning of the process the lime always absorbs a certain amount of sulphur irrespective of Whether slag is formed or not if the process is carried out in a converter. Removal of sulphur is, however, highly counteracted if the reaction takes place under oxidizing conditions. When carrying out the process in a rotating furnace the addition of lime is adjusted s-o that a not too viscous basic slag is obtained at the vbeginning of the treatment and thereafter the oxygen supply is cut of and the rotation is continued whereby it is possible to obtain a high degree of removal of sulphur. The sulphur containing slag m'ay then be drawn olf and the treatment continued. The good result obtained in this case also is due to the ability to rapidly'increase the temperature in the bath to a required extent by using a gas rich in oxygen -before too great a part of the more reducing elements in the pig iron and primarily the carbon has decreased too much whereby the succeeding equalization may be carried through under suicient reducing conditions which is essential if the sulphur is to .be removed completely or substantially completely. If the silicon content of the pig iron is high it may be preferable firstly to add a. small quantity of lime so that the initially formed slag will be acid and then draW olf this slag and add a new quantity of lime for the formation of a basic and sulphur binding slag. Also when the sulphur content of the pig iron is high it may be preferable to apply such a refining treatment in two steps. When using pure oxygen gas in the process the end product is necessarily always low in nitrogen. Itis, however, also possible when using nitrogen containing gas to obtain a substantially lower nitrogen content in the steel than is normally possible in a common Bessemer process or in the side blowing process, if during the carbon removal the air supply is cut off for a period and the rotation of the furnace is continued in neutral atmosphere. The refining then continues without supply of nitrogen and the nitrogen content of the bath is lowered.

The process may advantageously be carried out in a furnace shaped as a rotating drum. The oxygen containing gas may in this case be supplied at a relatively low velocity through a Water encaisser cooledtwyer introduced through the central part" of fone end Wall and the :reaction gases are drawn off through the corresponding` opening in the 'op` posite endwall. The iron treated and vpossibly also the slag may be drawn olf through an opening in the furnace lining which .is closed during the rotation. It is also possible to draw off the.

slag through the opening for reactiongases Aby tilting the furnace axially; The furnace may also be rotated insuch a position thatat least slag; maybe drawn oi `when the furnace.- is rotating.Y

This is especiallyofimportance because thusit.

is possible toY continuously drawy offslag duringr the process.

It must be possible'tovary the rotational speed of the `furnace within Wide Alimits as the most suitable rotational speed varies depending on the starting Amaterial and also on the altered chemf ical composition during the'process. Thus the formation of iron fumes is most-intense inthe beginning and. is counteracted'by a rapid stirring with accompanyingincreased rotational. speed. At the' end the iron losses in the slag increase and lalso arecounteracted by a rapid stirring.

Rapid` stirringis also often desirable from ai metallurgical point Aof view but involves increased Wear-of the lining. For decreasing this Wear it is forveconornical reasons recommendable to use an increased rotational speed only when necessary and to lower the speed when possible without disadvantage. Trials have shown that the inside wall ofthe furnace preferably is to be given a peripheral speed of at least 1 m per second, at

least during certain periods. In certain cases the best results have been obtained with a peripheral speed of .2 m per second or stillhigher..

When in" this specification the term pure oxygen gas. is used itv is intended to cover chemically pureoxygen gas with an oxygen content higherthan 99% as Well as technical pure oxygen gas With an oxygen content higher than about 85%. oxygenated air is intended to comprise air with anx oxygen content higher Vthan that vof Aatrnos The term air'enriched in oxygen or eter Was .about 150011mmL asiwas alsothe internal .1 length.: Theoxidation Wascarried out/.With a99:1A per cent ioxygen fgas which .was 'introduced intel thefurnacef and directed obliquely against the both cases was :about 30.'xninut'es` but theitotalf.A

furnace f tirnelr im: bthrcases -aamounted to4 .about one hourf dueto the 'often repeated'V samplingr In heat 1 20 cooledfscraplironfwas added-butin spite of athis :the vtemperaturefiat :the `Tend of the" treatmentwas normal. In heat 2 no scrap ironff was added for lowering Vthe bath temperature; as@ in this case the charge hada-tendency tobecome f too cold substantially due to the fact th'a't'the:A furnace -tat the 'beginning` of .the treatment had a lower; t temperature than ini vthe ir'st mentioned cases The most importantdffe'r'ence between the:` treatment ofthe two heats was that the fgas lpres- 1 sure' was vhigher 'for 4heat- :2 than.z 'for vheat A l "and "1 times higher thanfor heat P1; at. which the'speedlf Was "relatively 10W.'l

Pig iron; kgs Scrap iron; kgs; Burnt lime, kgs. Oxygen gas, kgs. Gas pressurefkgs/c Diameter of the tvvyernozzle,V inches Rotation speed of vthe furnace; -vrcvolutionsperminute'.;..-.

From the analysis v'and the 4course :or theA treats ment'fth'effollowing:may 1befstated':

Analysis" HEAT Yf1 Time from the M Analysis of the ironbath; Analysis of the slagi-A ginning oi the treatment iniminutes C Si Mn 12 il S N' -Fe M110 Sion Ca() t MgO PiOs pheric Vair and lower vthan the flower limit stated abovek for technical oxygen gas."

For illustrating the results obtainable according to the invention some data are given-from two heats -which were carried'out-in such' a way that they chemically correspond.'` vtothe Y- common basic Bessemer process. The heats were Atreated in the same furnace-whichwas rotatable and had the form .of va.-hcrizontal cylinder With-central openings in both end walls.- The internal diam'- The ironcontent intheslag-Was substantially 1' lowerin `heatn2 lthanin--vheat` -1 which is-due to more elevatedbasicitywhich partially isduetov thev lhigher basicity of` the-'slag but substantiallyv4 is due to the higher rotationalspeed. The iron 'Y content of :the slag in heat 2 -is `fullycomparable with' good results from :common ybasic Bessemeriron-obtained iii-converter blownfrom -thefbotatom.: -the--air is blown `from above against an cult to bring the phosphorus content below 0.050% without the iron content of the slag simultaneously rising to or above as shown by comparative trials. Due to the rotation, especially when using a high rotational speed, it is thus possible to obtain a substantial improvement in the iron yield which improvement may rise to 2-3% or even more if the slag quantities are great.

In heat 2 the surprising and important result was obtained that practically no iron fumes were formed in spite of the elevated pressure with which the oxygen gas in this case was blown against the bath. The rotation speed was thus suiicient for causing the necessary equalizing of the temperature. At heat 1 about 1% of the 'iron was gasiied involving a vigorous formation of fumes.

In spite of the long furnace time 20% scrap dolomite lining of the furnace was decomposed due to the rapid cooling and heating between the charges. At heat 1 the consumption of oxygen gas was considerably less than at heat 2 which is due to the lower gas pressure, which also in this respect is an advantage. The oxygen may as a rule be supplied at a pressure of 1.5 kgs/cm2 and it is in most cases possible to carry out the process if the pressure of the gas is only so high that the gas quantity blown into the furnace chamber is suiicient for completing the reiining process within a reasonable period of time.

As appears from the trials there is not obtained any shortening of the time necessary for the treatment. In this connection it is to be pointed out that a normal Bessemer process often runs too rapidly for the necessary control of the treatment conditions. In the method according to the invention it is possible to control the process completely and also to interrupt it by cutting ofi the supply of oxygen gas in which case the furnace atmosphere within a short time becomes neutral due to consumption of the oxygen so that only the iron bath and slag react with each other.

We claim:

l. A method of carrying out the treatment of iron alloys with oxygen-containing gases which comprises establishing a molten body of an iron" alloy in a reaction chamber which is rotatable about a substantially horizontal airis and is of substantially circular cross-section perpendicular to said axis, said body lling less than half oi said chamber and being at least partly covered by a molten slag, rotating said chamber for a period of time whereby said body is agitated but maintained unitary and introducing a gas which is free from combustible components and contains least 40 per cent of oxygen into the space in said chamber at a point above said body and discharging reaction gases from said chamber during at least a part of said period of rotation, the peripherial speed of said chamber during at least a part 10 of said period of rotation being at least l meter per second.

2. Method as defined in claim 1 in which the iron alloy is pig iron.

3. Method as dened in claim 1 in which the gas contains at least 50 percent of oxygen.

4. Method as dened in claim l in which the gas is technically pure oxygen.

5. Method as defined in claim 1 in which the gas is introduced into the chamber at a superatmcspheric pressure not greater than 1.5 lig/cm?.

6. Method as defined in claim 1 in which the alloy is pig iron and the iiow of gas is stopped before the decarburization of the pig iron is complete and the rotation of the reaction chamber is continued.

7. Method as dened in claim 1 in which the alloy is pig iron high in sulfur, the slag is a basic sulfur-binding slag and in which the introduction of gas is interrupted when the body is at desuliurizing temperature but still has a high content of carbon, continuing the rotation of the chamber until the desulfurizationis completed, removing the sulfur-binding slag, introducing a new slag and resuming the introduction of gas to effect decarburization of the pig iron.

6. Method as dened in claim 7 in which the iinal introduction of gas is stopped before the decarburization of the pig iron is complete and the rotation of the chamber is continued.

9. Method as dened in claim 1 in which the gas introduced into the chamber is diluted with gas of substantially the same composition as the reaction gases which are discharged from the reaction chamber prior to its introduction into the reaction chamber.

10. Method as deiined in claim 1 in which the gas introduced into the reaction chamber is intermingled with reaction gases Within the reaction chamber prior to contact thereof with the molten body.

BO MICHAEL STURE KALLING. OSKAR LENNART LINDSKOG.

CETED The following references are of record in the rile of this patent:

UNITED STATES PATENTS Number Name Date 76,581 Allen Apr'. 14, 1868 223,481 Chapin Jan. 13, 1880 411,417 Bookwalter Sept. 24, 1889 412,721 Bookwalter Oct. 15, 1889 855,603 Adams June 4, 1907 1,032,653 Brassart July 16, 1912 1,032,655 Brassert July 16, 1912 1,839,927 Neuhauss Jan. 5, 1932 2,068,785 Bain et al Jan. 26, 1937 2,074,164 Clair Mar. 16, 1937 2,093,666 Vogt Sept. 2l, 1937 2,110,066 Heuer Mar. l, 1938 2,209,158 Dillon July 23, 1940 FOREIGN PATENTS Number Country Date 2,207 Great Britain of 1858 12,950 Great Britain of 1901 456,548 Great Britain Apr. 5, 1922 462,837 Great Britain Mar. 17, 1937 514,352 Great Britain Jan. 24, 1938 

1. A METHOD OF CARRYING OUT THE TREATMENT OF IRON ALLOYS WITH OXYGEN-CONTAINNG GASES WHICH COMPRISES ESTABLISHING A MOLTEN BODY OF AN IRON ALLOY N A REACTION CHAMBER WHICH IS ROTATABLE ABOUT A SUBSTANTIALLY HORIZONTAL AXIS AND IS OF SUBSTANTIALLY CIRCULAR CROSS-SECTIONAL PERPENDICULAR TO SAID AXIS, SAID BODY FILLING LESS THAN HALF OF SAID CHAMBER AND BEING AT LEAST PARTLY COVERED BY A MOLTEN SLAG, ROTATING SAID CHAMBER FOR A PERIOD OF TIME WHEREBY SAID BODY IS AGITATED BUT MAINTAINED UNITARY AND INTRODUCING A GAS WHICH IS FREE FROM COMBUSTIBLE COMPONENTS AND CONTAINS AT LEAST 40 PER CENT OF OXYGEN INTO THE SPACE IN SAID CHAMBER AT A POINT ABOVE SAID BODY AND DISCHARGING REACTION GASES FROM SAID CHAMBER DURING AT LEAST A PART OF SAID PERIOD OF ROTATION, THE PERIPHERIAL SPEED OF SAID CHAMBER DURING AT LEAST A PART OF SAID PERIOD OF ROTATION BEING AT LEAST 1 METER PER SECOND 